1. Field of The Invention
This invention relates to a pneumatic sensor head for monitoring the thickness of a continuously moving sheet of material passing therethrough, and, more particularly, to a pneumatic sensor head which is capable of operating over wide variations in nominal thicknesses of monitored sheets of material.
2. Description Of The Prior Art
Prior art pneumatic sensor heads for monitoring the thickness of a continuously moving sheet of material, comprising first and second nozzles separated by a distance sufficient to receive the sheet material passing therethrough, are known. In these devices, a pressure signal is supplied to the nozzles. The back pressure from the nozzles is monitored, and provides an indication of the thickness of the material, since variations in back pressure are known to be proportional to variations in thickness of the material. However, in order to obtain predictable linear measurements, the clearance between each nozzle and the material passing between the nozzles must be less than ten percent of the diameter of the adjacent nozzle. Thus, a pneumatic sensor head, which is comprised of a pair of aligned nozzles that are separated by an unadjustable fixed distance, is only useful to monitor material within a very limited range of a nominal material thickness. This causes a business operation to stock a large number of sensor heads, wherein each sensor head would be used to monitor the thickness of a sheet of material over a specified nominal range. The stocking of sensor heads for the above purpose is obviously impractical and expensive.
In order to overcome the above disadvantage of having to stock a plurality of sensor heads in order to monitor sheet material over a wide nominal thickness range, sensor heads have been developed which allow the nozzles to be movable to enable the separation therebetween to be adjusted with the use of a feeler gauge. The use of a feeler gauge has a disadvantage in that each time the gap between the nozzles of a sensor head is readjusted to be the same as a previous nominal thickness setting, the latter monitored nominal thickness measurement may not be identical to the previous monitored nominal thickness measurement, since feeler gauge adjustments can vary and are not very reliable. Furthermore, adjustable nozzles require flexible tubing or connections, and if the connections are loosened and slip, the nozzle setting can change. This is particularly true under conditions where the sheet of material may inadvertently be repeatedly impacting with one or both nozzles.